The present invention relates to structures that enable electromagnetic beam transformation or spot size conversion between a large-mode-size and a small-mode-size, and methods of making the same. In particular, the present invention relates to optical structures having an effective Graded Refractive Index (GRIN) distribution in the vertical direction perpendicular to the optical axis of light propagation, and a lens shaped optical interface structure perpendicular to the direction of light propagation.
In photonics chip industry, one of the basic processes is the transformation of a light beam from a large size to a small size (or vice versa) between two optical waveguides of different mode sizes. However, the light coupling efficiency between the two optical waveguides involved in this basic process is generally low. There are a few commonly used approaches in the present state of the art to make a lens structure that addresses the above needs.
One approach involves using the property of light refraction at an optical interface. By making an optical medium such as a glass into a certain shape such as a ball or sphere, a lens can be made to focus a light beam. U.S. Pat. No. 6,026,206, titled “Optical coupler using anamorphic microlens”, is an example of one such approach.
Another approach to address the needs is to create a GRIN distribution of an optical medium. Due to the GRIN distribution, the light beam bends as it travels inside the medium. This property is used in achieving light focusing. A commonly used example is an axially symmetric GRIN rod lens, which is used for collimating a light beam emitted from a single mode fiber. U.S. Pat. No. 6,267,915, titled “Production Method for Objects with Radially-Varying Properties” U.S. Pat. No. 6,128,926, titled “Graded Index Lens for Fiber Optic Applications and Technique of Fabrication” and U.S. Pat. No. 6,172,817, titled “Graded Index Lens for Fiber Optic Applications and Technique of Fabrication”, are a few examples of this approach.
For coupling of light between a single mode fiber and a semiconductor waveguide based device (such as a semiconductor laser), the most commonly used coupling optics is a lensed fiber. Such a lensed fiber is made by shaping the end of the fiber into a hemispherical or cylindrical lens using lapping and polishing and/or melting means. U.S. Pat. No. 5,845,024, titled “Optical fiber with lens and method of manufacturing the same”, and U.S. Pat. No. 6,317,550, titled “Lensed optical fiber” elaborate upon such optical fiber with lens.
However, there are a few problems associated with the above-mentioned lenses. For a symmetric lens element, such as a GRIN rod lens, or a ball lens or a tapered conical lensed fiber, the focused mode profile from a circular optical fiber is circular. As a semiconductor waveguide almost always has an elliptical mode profile, there is a large mode mismatch, which inherently results in low light coupling efficiency. Thus, the coupling efficiency for coupling light between a single mode fiber and a semiconductor waveguide based device cannot be very high. In fact, the coupling efficiency is only as high as about 80% for such lenses.
For reducing the problem of mode mismatch and subsequently increasing light coupling efficiency, wedge fibers can be used. There are generally two kinds of wedge fibers: single wedge fiber and double wedge fibers. Single wedge fibers have an elliptical focused beam spot with the long horizontal axis spot size basically equal to that of the circular single mode fiber spot size. However, as the horizontal axis mode spot size of a semiconductor waveguide (such as a laser diode) is typically only about 3 to 4 μm, and the beam spot size of a single mode fiber is about 6 to 10 μm, there remains a mismatch in the horizontal mode size. A double wedge fiber addresses the problem of mismatch in horizontal mode size. Using a double wedge fiber, the horizontal mode size can be made to match that of a semiconductor waveguide. However, the vertical spot size cannot be made to match with that of a semiconductor waveguide.
The above disadvantage of vertical spot size mismatch is present in all optical-interface-refraction based lenses (including those described above). This is due to the fact that the minimum vertical spot size for these lenses is about 1.5 μm (for the near infrared optical communication spectrum region) while the typical vertical mode size of a semiconductor waveguide is about 1 μm. In addition, for these lenses, especially lensed fibers, there is a large variation in the radius of curvature of the lens because each lens is made individually one at a time through processes, such as arcing or laser melting, that cannot guarantee high precision consistency. Thus, all the above-mentioned lenses will have a relatively low coupling efficiency, and there is a low consistency in the coupling efficiency.
Thus, what is needed in the photonics chip packaging industry is a superlens that can provide a focused beam spot size, and can independently achieve horizontal and vertical phase and/or wave-front matching with those of a semiconductor waveguide. In addition, the vertically focused spot size must be of the order of about 1 μm in order to match with that of a typical semiconductor waveguide. Only by doing so can a light coupling efficiency well above 80% become practically achievable.